The purpose of this research is to determine the cytological effect of the gonadotropin, human chorionic gonadotropin (HCG), on the Sertoli cells of the canine testis. Known concentrations of HCG will be perfused in vivo into a vascularly isolated testis of anesthetized adult and prepubertal dogs and the contralateral testis will serve as control. The testes will be fixed in situ by perfusion with formaldehyde-glutaraldehyde-picric acid (FGP). Freeze-fracture replicas, as well as routinely prepared specimens will be examined by transmission electron microscopy. Scanning electron microscopic studies will be done to demonstrate the three-dimensional relationship of the cells within the seminiferous tubules of treated and control animals. Lanthanum nitrate, an electron dense tracer, will be used to determine the patency of the Sertoli: Sertoli tight junction. Tight junctions within the complex junction between Sertoli cells represent the main component of the blood-testis barrier. The establishment of this barrier effectively isolates the haploid germinal cells from the interstitial fluids. The results of preliminary experiments indicate that bands of tight junctions form around the Sertoli cells before the initiation of spermatogenesis. How the tight junctions between Sertoli cells are modified to permit the ascent of the developing germinal elements from the basal tubular compartment toward the lumen is not known. However, the discovery of septate junctions, a type of junction not previously demonstrated in vertebrate tissue, within the complex Sertoli junction may offer a clue to the development of the tight junction. HCG cytologically alters the Sertoli:Sertoli complex junction in the dog. A study of this alteration may result in a better understanding of the formation and control of the Sertoli:Sertoli junction and the role of the Sertoli cell plays in the process of spermatogenesis. From a medical viewpoint this information could be instrumental in the development of new approach to the regulation of fertility and the treatment of sterility.